In using rotary dobbies the movements of the loom heddle frames are obtained by linkages operated by the main rotary shaft of the dobby which for every half revolution must undergo a pause during which keys can be inserted, withdrawn, or left inserted or left withdrawn, in order for said main shaft to be secured to or released from, or kept secured to or kept released from, the thrust rods which each control the movement of one heddle frame of the loom in accordance with a predetermined program. The rotary dobby control modulator is precisely that member which generates said keying pauses by converting the continuous motion of the dobby drive shaft into a motion comprising two pauses per revolution of the dobby main shaft.
Various types of modulators for rotary dobby control are known from the state of the art.
In one of these known types, said two pauses are obtained by two superposed conjugate cams fixed to the dobby casing coaxial to the drive shaft and main dobby shaft, and each with a contour having an axis of asymmetry (ie., they are specularly antisymmetrical), the cams cooperating with at least one system consisting of two wheels acting in the two different planes of said conjugate cams and carried by a rocker support comprising a toothed sector which engages with a gear wheel rigid with the dobby main shaft, said rocker support being hinged on a disc rigid with the dobby drive shaft.
In this manner the particular form of the conjugate cams means that for each revolution of the dobby drive shaft, said drive shaft and the dobby main shaft can undergo a relative movement such that the main shaft rotates intermittently with two short pauses every revolution, whereas the drive shaft continues to rotate at uniform angular velocity.
Such a construction has however two drawbacks the first of which is due to the fact that during the relative rotation between the toothed sector and the main shaft gear wheel the various teeth undergo a multiplicity of mutual contacts and variations in such contacts due to the clearances and the irregularity between the toothing, thus causing inaccuracies and harmful vibration during the intermittent rotation of the main shaft, such vibration becoming amplified and affecting the movement of the cam portions of the dobby thrust rods, and finally affecting the movement of the heddle frames, to thus affect the correct operation of the loom and result in frequent breakages of the yarns supported by said frames. The other drawback is that said toothed sectors are supported cantilevered on the disc rigid with the dobby drive shaft so that the flexure caused by the considerable forces acting on the hinge pins of said toothed sectors results in incorrect modulator operation because the contour of the conjugate cams is not perfectly followed, and as the effect increases with rotational speed the result is a further limitation on the use of this type of modulator, which is unsuitable for high-speed looms. Said drawbacks are lessened in another type of known modulator in which two rollers cooperating in different planes with the conjugate cams are carried by a rocker support which is hinged between a disc and a counter-disc rigid with the dobby drive shaft and comprises a guide slot in which a slider slides, pivoted on an appendix jutting from the main shaft.
In this manner the multiplicity of contact of the teeth is replaced by the sliding contact of the slider and the rotation of the hinge pin of the slider itself, and in addition the arrangement of the rocker supports, which are no longer cantilevered but rock about a shaft fixed at its two ends to rigid supports, obviates said drawback deriving from flexure. In this respect, whereas the teeth have very small contact surfaces and are subjected to correspondingly large variable hertzian pressure during the various engagement stages with consequent variable deformation leading to system vibration, the slider has a large sliding contact surface with consequent small deformation and relatively small clearances, to thus represent an assembly of greater operating precision and causing lesser vibration in the movement of the loom heddle frames, leading to lesser yarn breakages and fewer loom stoppages, with a smaller time lost for repairs.
However even this type of modulator has limitations in the case of operation at high loom speeds exceeding 500 beat-ups per minute as required for modern dobbies, because of the inevitable clearances and the constructional inaccuracies present in it.
In this respect it is well known that the damaging vibration is generated because of the clearances and increases exponentially with speed.
To increase the operating speed it is therefore necessary to reduce motion irregularities and vibration, and therefore to reduce the clearances present in the modulator to a minimum by improving the quality of its construction or machining. In said known modulator, a pin and slider are used to transmit motion from the rocker support to the main shaft. The loads transmitted are the same for the pin and slider, but this latter, because it contains the hinge pin in its interior, has to have fairly large transverse dimensions of at least two or three times the pin diameter, and therefore has at least two or three times the clearance of the pin because of the fact that the clearance required for mobile parts is proportional to their dimensions and their level of finishing. Again, as the flat slide surfaces of the slider must be able to easily slide along a guide and lubrication must be provided for, the clearances cannot be kept too narrow otherwise the oil cannot flow, and the slider becomes more inclined to seize because said flat surfaces easily tend to incline to the extend of the guide clearances, and to jam at the two opposing end corners, with the result that the clearances must be made even larger. To all this must then be added the intrinsic difficulties in the construction of flat surfaces such as those of the said slider and in particular of its guide, which can not be given the same level of finishing as that obtainable for a pin and its guide, for which precision grinding can be used for the pin and lapping can be used for the guide, resulting in considerable constructional precision.